1. Field of the Invention
The invention relates to a process for preparing tert-butanol (TBA) by reacting isobutene-containing hydrocarbon mixtures with water over acidic solid catalysts in a plurality of reactors, wherein the conversion is increased upstream of the last reactor by removing a portion of the TBA present in the reaction mixture.
2. Description of the Background
Tert-butanol (TBA) is an important product prepared on the industrial scale and is used as a solvent and as an intermediate for the preparation of methyl methacrylate. It is a precursor for the preparation of peroxides such as peroxy ketals, peresters or dialkyl peroxides, having at least one tertiary butyl group. These compounds are used as oxidizing agents and as initiators for free-radical reactions, for example olefin polymerization or crosslinking of plastics. Tert-butanol is used as an intermediate for obtaining pure isobutene from isobutene mixtures. In addition, it is a reagent for introducing tertiary butyl groups. Its alkali metal salts are strong bases which find use in many syntheses.
Tertiary butanol can be obtained by acid-catalyzed addition of water to isobutene. Technical-grade isobutene mixtures frequently also contain other olefins, for example 2-butenes. Industrial processes therefore employ conditions in which almost exclusively the isobutene but not the other olefins is hydrated, and side reactions such as homo- or heterooligomerization of the olefins are virtually fully suppressed. Such processes commonly operate in the liquid phase and can be divided into two groups: a) processes in which the conversion is effected in an aqueous catalyst solution and b) heterogeneous catalytic processes in which solid catalysts which are insoluble in the reaction phase are used.
The hydration of isobutene to tert-butanol with the aid of solid acidic catalysts which are soluble neither in the reactants nor in the products has the advantage that the reaction mixture is acid-free and can be worked up to tert-butanol without losses by dissociation or by other side reactions. The reaction proceeds on the surface of the catalyst. So that a reaction occurs, both reaction partners have to be at the active site of the catalyst at the same time. This is complicated by water and isobutene or an isobutene-containing hydrocarbon mixture not being mutually miscible. In order to obtain acceptable conversions, solvents are used which enable a homogeneous mixture of water and isobutene starting mixture.
DE 30 31 702 A1 describes methanol for this purpose as a solvent both for water and for isobutene or for an isobutene-containing hydrocarbon mixture. The products obtained are both tert-butanol and methyl tert-butyl ether.
In EP 0 010 993 A1, aliphatic carboxylic acids having from 1 to 6 carbon atoms are used as the solvent for both reactants. The by-products formed are the tertiary butyl esters of these acids. These have to be hydrolyzed to tert-butanol and carboxylic acids.
In DE 30 31 702 A1, sulfolanes are used, and, in U.S. Pat. No. 4,327,231, neo-type polyhydric alcohols, for example neopentyl glycol. These solvents have to be removed from the tert-butanol. In addition, there is the risk that the solvent used is destroyed in the long-term operation of such a plant.
In order to avoid these disadvantages, the target product, TBA, is used as solubilizer in some processes. Such processes are described, for example, in WO 99/337755 or DE 30 25 262. In these processes, a mixture of a hydrocarbon fraction which comprises isobutene, TBA and water is converted in a reactor battery over acidic catalysts arranged in a fixed bed. The first reactor is usually operated in loop mode and the other in straight pass. Upstream of each further reactor, water of reaction may be metered in. The effluent of the last reactor is separated by distillation into a hydrocarbon mixture comprising the unconverted isobutene and crude TBA. A portion of the crude TBA is recycled into the first reactor. The other portion may be used as such or worked up to TBA and/or TBA/water azeotrope.
In these processes, the TBA content increases and the isobutene content decreases from reactor to reactor as a consequence of the progress of the reaction. The composition of the reaction mixture approaches, at a decreasing rate, the thermodynamic equilibrium between water, isobutene and TBA, so that complete conversion cannot be achieved. Starting from technical-grade isobutene streams, for example raffinate I, conversions of only approx. 82% are achieved.
In the application DE 102 60 991 which was yet to be published at the priority date of the present application, a process is described for preparing TBA in which a relatively complicated reactive distillation is used.